A receiver, also known as User Equipment (UE), mobile station, wireless terminal and/or mobile terminal is enabled to communicate wirelessly in a wireless communication system, sometimes also referred to as a cellular radio system. The communication may be made, e.g., between two receivers, between a receiver and a wire connected telephone and/or between a receiver and a server via a Radio Access Network (RAN) and possibly one or more core networks.
The receiver may further be referred to as mobile telephones, cellular telephones, computer tablets or laptops with wireless capability. The receivers in the present context may be, for example, portable, pocket-storable, hand-held, computer-comprised, or vehicle-mounted mobile devices, enabled to communicate voice and/or data, via the radio access network, with another entity.
The wireless communication system covers a geographical area which is divided into cell areas, with each cell area being served by a transmitter, also referred to as a radio network node or base station, e.g., a Radio Base Station (RBS), “eNB”, “eNodeB”, “NodeB” or “B node”, depending on the technology and terminology used. Sometimes, also the expression cell may be used for denoting the transmitter/radio network node itself. However, the cell is also, or in normal terminology, the geographical area where radio coverage is provided by the transmitter/radio network node at a base station site. One transmitter, situated on the base station site, may serve one or several cells. The transmitters communicate over the air interface operating on radio frequencies with the receivers within range of the respective transmitter.
In some radio access networks, several transmitters may be connected, e.g., by landlines or microwave, to a Radio Network Controller (RNC), e.g., in Universal Mobile Telecommunications System (UMTS). The RNC, also sometimes termed Base Station Controller (BSC), e.g., in GSM, may supervise and coordinate various activities of the plural transmitters connected thereto. GSM is an abbreviation for Global System for Mobile Communications (originally: Groupe Spécial Mobile). In 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE), transmitters, which may be referred to as eNodeBs or eNBs, may be connected to a gateway, e.g., a radio access gateway, to one or more core networks.
In the present context, the expressions downlink, downstream link or forward link may be used for the transmission path from the transmitter to the receiver. The expression uplink, upstream link or reverse link may be used for the transmission path in the opposite direction, i.e., from the receiver to the transmitter.
In order to enable coherent demodulation of data, the transmitter has to send a pre-defined reference signal, or pilot signal as it also may be referred to as, to the receiver/UE. The reference signal may not encode any information and it is typically known to the receiver. From the reference signal, using a priori information on its modulation symbols and time-frequency location, the receiver may, based on the received reference signal, obtain channel estimates such as, e.g., the phase and amplitude of the channel frequency response, which are used for channel equalization prior to the demodulation.
In the prior art 3GPP LTE system, multiple transmit and receive antennas are supported and the notion of antenna port is used. Each downlink antenna port is associated with a unique reference signal. An antenna port may not necessarily correspond to a physical antenna and one antenna port may be mapped to more than one physical antenna. In any case, the reference signal may be used for channel estimation for data that is transmitted on the same antenna port. Channel estimation therefore needs to be performed for all antenna ports that are used for the data transmission. A number of reference signals have been defined in the LTE downlink, e.g., Common Reference Signal (CRS).
CRS is a cell-specific reference signal, which is transmitted in all subframes and in all Resource Blocks (RBs) of the carrier. The CRS serves, among several purposes, as a reference signal for phase and amplitude reference for coherent demodulation, i.e., to be used in channel estimation. Up to 4 antenna ports (labelled 0-3) may be accommodated with the CRS. These antenna ports are multiplexed on orthogonal time-frequency resources, i.e., disjoint sets of Resource Elements (REs). The CRS may offer robustness as it supports transmit diversity based PDSCH transmission.
The RE is the smallest time-frequency entity that can be used for transmission in LTE, and may convey a complex-valued modulation symbol on a subcarrier. In this context, the RE may be referred to as a time-frequency resource. The RB comprises a set of REs or a set of time-frequency resources and is of 0.5 ms duration (e.g., 7 Orthogonal Frequency-Division Multiplexing (OFDM) symbols) and 180 kHz bandwidth (e.g., 12 subcarriers with 15 kHz spacing). The LTE standard refers to a Physical Resource Block (PRB) as a RB where the set of OFDM symbols in the time-domain and the set of subcarriers in the frequency domain are contiguous.
With multiple antennas, it is possible to achieve beamforming by applying different complex-valued weights on the different antenna ports, also referred to as precoding. However, since the CRS is cell-specific, it cannot be receiver-specifically precoded, i.e., it cannot achieve any beamforming gains although the user data channel may undergo beamforming since it is not cell-specific. Therefore, typically the precoder used for the data channel has to be signalled to the receiver.
Another defined reference signal is the Demodulation Reference Signal (DM-RS). This is a receiver-specific reference signal and it is only transmitted in the resource blocks and subframes where the receiver has been scheduled data i.e., containing the Physical Downlink Shared Channel (PDSCH). Up to 8 antenna ports may be accommodated by the DM-RS. The antenna ports (labelled 7-14) are multiplexed both in frequency and by orthogonal cover codes in time. Since it is receiver-specific, the DM-RS may be precoded with the same precoder used for the PDSCH, hence beamforming gains may be achieved for the reference signal. Since both data channel and the DM-RS use the same precoder, the precoding becomes transparent to the receiver. Thus there is no need to signal the precoder to the receiver as it can be regarded as part of the channel, which is estimated by the DM-RS.
In order to receive the PDSCH, the receiver is monitoring a set of time-frequency resources i.e., Control Channel Elements (CCEs) or Enhanced CCEs (ECCEs) in a downlink control channel such as e.g., PDCCH or EPDCCH and performs blind decoding to detect Downlink Control Information (DCI) associated with the PDSCH transmission. The receiver is configured in one of several transmission modes wherein it is monitoring one DCI format (e.g., DCI format 1A) which typically may be used when a robust transmission of the PDSCH is needed, e.g., using transmit diversity. DCI format 1A schedules the PDSCH on antenna port 0, or 0, 1 or 0, 1, 2, 3, with the exception in MBSFN subframes where antenna port 7 is used. In addition, the receiver monitors one additional DCI format, which may utilise DM-RS for PDSCH demodulation. This additional DCI format can typically accommodate much more advanced transmission schemes such as Single User MIMO (SU-MIMO) or Multi User MIMO (MU-MIMO) or CoMP transmission.
The antenna port to be assumed by the receiver, based on CRS or DM-RS, for demodulating the PDSCH is determined from the detected associated DCI format depending on the configured transmission mode. In some cases, the DCI format itself may also contain additional bits related to which of the DM-RS antenna ports (e.g., port 7 or 8) that should be used. This is, e.g., applicable when MU-MIMO is used. The prior art LTE system does not provide any dynamic switching between using cell-specific reference signals or receiver-specific reference signals.
In order to improve the spectral efficiency of the 3GPP LTE system, it has been considered to define a new carrier type which only transmits the CRS in a subset of the subframes in a radio frame and possibly also in a subset of the resource blocks of the carrier. A further overhead reduction could also be envisaged by only utilising one CRS, i.e., antenna port 0. This reduced CRS would not be used for channel estimation but only for time- and frequency synchronization and measurements. PDSCH demodulation would thus primarily be based on the DM-RS.
FIG. 1 shows a non-limiting example of a subframe for a carrier with 14 resource blocks where a cell-specific reference signal is transmitted in resource block 2-11, which may in other examples occupy all resource blocks (e.g. 0-13). User-specific reference signals may in this example be transmitted in resource block 0, 1, 2, 3, 10, 11, 12 and 13.
However, in the prior art LTE system, the user-specific reference signals may comprise time-frequency resource elements (REs) overlapping with the synchronization signals or the broadcast channel. This implies that DM-RS based PDSCH transmission cannot be accommodated in such resource blocks. The six central resource blocks may, depending on subframe number, contain synchronization signals and a broadcast channel. In one example the reduced CRS would be transmitted in subframes where DM-RS overlaps with at least a synchronization signal. In other subframes, where synchronization signals and/or broadcast channels are not transmitted the reduced CRS may not even be present at all and the DM-RS may be utilised in all resource blocks. Thereby, subframes wherein all transmissions are based on the DM-RS would occur. There would therefore necessarily have to be a DCI format (similar to DCI format 1A) which schedules the PDCSH on DM-RS ports only.
A system is considered wherein, for at least one subframe, a user-specific reference signal can be transmitted only in a subset resource blocks. The system further includes a cell-specific reference signal which is applicable for channel estimation for data channel demodulation. The data channel may thus be transmitted either by the user-specific reference signal or the cell-specific reference signal.
A first problem comprises determining which reference signal (antenna port) that should be utilised.
A second problem comprises determining which resource blocks of a data channel assignment that should be used.
In the prior art LTE system, DM-RS based PDSCH transmission is not supported in resource blocks where the DM-RS would overlap with a synchronization signal or a broadcast channel. CRS-based PDSCH transmission is supported in all resource blocks. The designated antenna port is given by the configured transmission mode, subframe type (i.e., normal subframe or MBSFN subframe) and, for some instances of DM-RS, additionally with explicit bits in the corresponding DCI format.
In the prior art LTE system, both CRS and DM-RS can be transmitted, which leads to high reference signal overhead, decreased throughput and reduced overall system efficiency. It is a further objective to maximize the flexibility for the system to select a suitable reference signal (antenna port) for a given transmission while at the same time not requiring overhead signalling for informing the receiver about the selected antenna port.
Hence, it is a problem to assure that there is a reasonable trade-off between reference signal overhead and performance.